Electrolytic capacitors of various types are in widespread use in the electronics industry as a result of their desirable electrical characteristics over a relatively wide temperature range, compact size, low cost and long life. Metal foil electrolytic capacitors and sintered, powder metal electrolytic capacitors using either a liquid or a solid electrolyte are described by Walter J. Bernard in an article entitled "Developments in Electrolytic Capacitors", Journal Of The Electrochemical Society, Vol. 124-No. 12, December 1977.
These electrolytic capacitors achieve high capacitance per unit volume or weight by virtue of an extremely thin dielectric oxide film formed on the anode electrode surface in conjunction with an extremely high anode electrode surface area. As is well known, the capacitance of the electrolytic capacitor varies inversely with the thickness of the dielectric oxide film and directly with the surface area of the anode electrode. The thin dielectric oxide film is typically formed on the anode electrode surface by an electrochemical anodizing treatment in a suitable acid or base solution as is well known; e.g., see the article by Donald M. Trohe entitled "Capacitors", Scientific American, July, 1988, pp. 86-90 and another article by W. J. Bernard et al entitled "Anodic Oxidation Of Porous Aluminum Pellets", Electrocomponent Science and Technology, Vol. 1, 1974, pp. 59-64 (Gordon and Breach Science Publishers Ltd).
Considerable effort has been expended to increase anode electrode surface as a way to increase the capacitance of electrolytic capacitors. In the metal foil type of electrolytic capacitor, high anode electrode surface areas have been achieved through chemical or electrochemical etching of the foil electrode surface. On the other hand, the sintered, metal powder electrolytic capacitor has relied on the porous nature of the sintered, powder metal anode electrode to provide a large electrode surface area. In particular, the anode electrode comprises a body made by compressing a metal powder/binder mixture to a desired electrode shape and then sintering the compressed body at elevated temperature to sinter the metal powder particles together and burn out the binders. Removal of the binder leaves a highly porous electrode body having a tremendous particulate surface area exposed within the body to act as an electrode surface.
Although the capacitor industry has been successful in increasing the capacitance of electrolytic capacitors with such developments as the aforementioned anodized, etched foil electrode element and anodized, sintered powder metal electrode element, there is a continuing desire in the industry for a capacitor electrode element that exhibits a large electrode surface area amenable to anodizing treatments to form a thin, dielectric oxide film thereon and that offers the possibility of achieving still further increases in the capacitance of an electrolytic capacitor without corresponding increases in capacitor size.